Rankine cycle system and method

ABSTRACT

A Rankine cycle system and method is described and illustrated, and in some embodiments includes an expander, a pump, a condenser, and a receiver comprising a variable fluid volume at least partially defined by a movable member, wherein the variable fluid volume defines at least a portion of the working fluid flow path between the condenser and the inlet of the pump. Also, a method of charging a Rankine cycle system with working fluid is described and illustrated, and can include applying a regulated pressure to a chamber located within a receiver, introducing the working fluid to the Rankine cycle system, the working fluid being separated from the chamber by a movable member of the receiver, monitoring displacement of the movable member, and stopping the introduction of working fluid into the Rankine cycle system when the movable member reaches a predetermined position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 61/407,119, filed Oct. 27, 2010, the entire contents of which are hereby incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No. W909MY-09-C-0084 awarded by the US Army, Research, Development and Engineering Command. The government has certain rights in the invention.

BACKGROUND

Vapor power cycles are known in the art as a means whereby heat energy can be converted to useful mechanical work. One such cycle, commonly known as the Rankine cycle, utilizes a working fluid that is cyclically pressurized from a low-pressure liquid state, vaporized using a heat source, non-adiabatically expanded to a low-pressure vapor, and cooled and condensed back to the low-pressure liquid state. In cycles of this kind, mechanical energy is produced during the non-adiabatic expansion of the vapor.

Rankine cycle systems may be especially beneficial in recovering energy from waste heat streams. Such waste heat stream may, for example, be found in the exhaust of a combustion process such as a burner or a combustion engine. In some applications, a Rankine cycle waste heat recovery system can be advantageously employed on a vehicle to recover waste heat from the exhaust of the internal combustion engine powering the vehicle. As vehicle efficiency standards are pushed progressively higher, such waste heat recovery systems become a means by which the required efficiency targets can be achieved.

A variety of working fluids may be suitable for use with Rankine cycle systems. As one example, water is commonly used as a working fluid in steam turbines operating on a Rankine cycle. For certain applications, however, other fluids may be preferable.

An undesirable condition can sometimes occur in Rankine cycle systems when the working fluid is cooled to a temperature that corresponds to a saturation pressure that is lower than the atmospheric pressure. Preventing the infiltration of ambient air into the system can be exceedingly difficult with the resulting pressure gradient, and the presence of such non-condensable gases into the working fluid volume can cause the Rankine cycle system to operate in a sub-optimal fashion, or even to not operate at all.

SUMMARY

According to some embodiments of the invention, a Rankine cycle system includes a pump, an expander, and a condenser located along a working fluid flow path between an outlet of the expander and an inlet of the pump. The system also includes a receiver comprising a variable fluid volume at least partially defined by a movable member, wherein the variable fluid volume defines at least a portion of the working fluid flow path between the condenser and the inlet of the pump.

In some embodiments, the system includes a liquid sub-cooler located along the working fluid flow path between the receiver and the inlet of the pump. The condenser and the liquid sub-cooler may be parts of a single heat exchanger in certain embodiments.

In some embodiments, at least a portion of the variable fluid volume defines a cylindrical volume, with the movable member defining an end of the cylindrical volume. The movable member may be movably disposed within the receiver so as to vary the length of the cylindrical volume.

According to some embodiments, the receiver further comprises a second variable fluid volume at least partially defined by the movable member. In some such embodiments the second variable fluid volume is in fluid communication with the environment to maintain a substantially atmospheric pressure within the second variable fluid volume, while in other such embodiments the second variable fluid volume is in fluid communication with a regulated pressure source.

In some embodiments, the receiver includes a position indicator to indicate the location of the movable member within the receiver. The position indicator may, in some embodiments of the invention, be magnetically coupled to the movable member.

According to some embodiments, the variable fluid volume is further defined by an end face of the receiver. The end face can include first and second ports in fluid communication with the variable fluid volume, the ports defining portions of the working fluid flow path immediately upstream and downstream of the variable fluid volume. In some embodiments, at least one of the end face and the movable member includes an offset surface so that a fluid flowpath is maintained between the first and second ports when the offset surface is in contact with the other of the end face and the movable member.

Some embodiments of the invention provide a method of charging a Rankine cycle system with working fluid, including the steps of: applying a regulated pressure to a chamber located within a receiver in the Rankine cycle system; introducing the working fluid to the Rankine cycle system, the working fluid being separated from the chamber by a movable member in the receiver; monitoring the displacement of the movable member; and stopping the introduction of working fluid into the Rankine cycle system when the movable member reaches a predetermined position. In some embodiments, the regulated pressure is greater than the saturation pressure corresponding to the temperature of the working fluid.

These and other aspects of the invention will become apparent to one of ordinary skill in the art upon inspection of the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a Rankine cycle system according to an embodiment of the present invention.

FIG. 2 is a perspective view of a receiver according to an embodiment of the present invention.

FIG. 3 is a sectional perspective view of the receiver of FIG. 2, taken along lines 3-3 of FIG. 2.

FIG. 4 is a perspective view of a receiver according to another embodiment of the present invention.

FIG. 5 is a perspective view showing the interrelation of certain components of the system of FIG. 1 according to an embodiment of the invention.

FIG. 6 is a graph of saturation pressure vs. temperature for two different fluids.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.

A Rankine cycle system 1 according to an embodiment of the present invention is depicted in schematic fashion in FIG. 1. The movement of a working fluid through the system 1 is illustrated by the solid black arrows. As shown, a pump 2 directs the working fluid through the system 1 in a closed loop. Energy in the form of heat is received by pressurized liquid working fluid in a heating section 4, and a portion of that energy is removed from the working fluid and converted to useful work in an expander 3. The heating section 4 may include one or more heat exchangers through which the working fluid can be directed in order to vaporize the pressurized liquid working fluid, thereby converting it to a pressurized vapor state. The heat so transferred in the heating section 4 may in some cases be waste heat contained in an exhaust stream of a process such as, for example, a combustion process. In other cases, the heat may be derived from other sources.

The working fluid can be any type of fluid that may be advantageously used in such cycles. By way of example only, the working fluid may include: water; ammonia; alcohols, including but not limited to ethanol and methanol; refrigerants, including but not limited to R134a, R152a, and R22; hydrocarbons, including but not limited to propane and butane; organic working fluids, including but not limited to R245fa; and combinations thereof.

It should be understood that the pump 2 can be any type of machinery used to pressurize and provide motive flow to a liquid, including but not limited to positive displacement pumps, reciprocating action pumps, rotary action pumps, kinetic pumps and centrifugal pumps, among others. Similarly, the expander 3 can be any type of machinery capable of converting pressure energy into mechanical work, including but not limited to piston expanders, impulse turbines and reaction turbines, among others.

Continuing with the description of the system of FIG. 1, the working fluid exits the expander 3 as a low-pressure (relative to the pressure at which it enters the expander 3) vapor, and is returned to the pump 2 inlet by way of a flowpath 9. Along the flowpath 9, residual heat is rejected from the working fluid in a condenser 5, thereby converting the low-pressure vapor to a low-pressure liquid. Additionally, a receiver 6 is located along the flowpath 9 between the condenser 5 and the inlet of the pump 2. A sub-cooler 7 may optionally be included between the receiver 6 and the pump inlet in order to reject additional heat from the working fluid. In some embodiments, however, the sub-cooler 7 may not be included, or it may be incorporated into the condenser 5.

The illustrated receiver 6 contains a movable member 8 that at least partially defines a chamber 10 within the receiver 6. The chamber 10 comprises a portion of the flow path 9 between the exit of the condenser and the inlet of the pump 2, so that the working fluid traveling along the flowpath 9 passes through the chamber 10 between the condenser 5 and the pump inlet.

One embodiment of a receiver 6 is shown in greater detail in FIGS. 2 and 3. The receiver 6 as shown therein includes a port 20 whereby working fluid can enter into the chamber 10, and a port 21 whereby working fluid can exit the chamber 10. An additional optional port 22 is also shown. This additional port 22 may be adapted to a number of uses, including connection of a pressure or temperature sensor, venting non-condensable gases from the receiver 6, providing a pressure relief vent, and providing a convenient working fluid charging port for the waste heat recovery system 1.

The illustrated receiver 6 as shown in FIGS. 2 and 3 additionally includes a cylindrical shell 12 extending between first and second end caps 13, 14. The first and second end caps 13, 14 are in turn connected to first and second end plates 15, 16 respectively. These connections may be accomplished by way of mechanical fasteners (e.g. screws or the like), or by welding, soldering, brazing, or other known processes. In some cases, the first end plate 15 may be integral to the first end cap 13, and similarly the second end plate 16 may be integral to the second end cap 14. The shell 12 is sealingly attached to the end caps 13, 14 by way of o-rings 19. Tie rods 17 extend between the end plates 15, 16 in order to maintain assembly between the shell 12 and the end caps 13, 14, and to resist the pressure that may be exerted by the working fluid during operation.

As best seen in the sectional view of FIG. 3, the movable member 8 may be embodied as a free piston. The chamber 10 is located between the movable member 8 and the first end cap 13, whereas the chamber 11 is located between the movable member 8 and the second end cap 14. The movable member 8 is disposed within the shell 12 so as to be movable in sliding fashion between the end plates 13, 14. O-rings 18 are positioned within grooves in the free piston comprising the movable member 8 in order to provide leak-free separation between the chambers 10, 11.

In other embodiments, the illustrated movable member 8 may be replaced with another structure or device suitable for adjusting the volume of the chamber 10. For example, in some embodiments, the movable member may include a diaphragm that deflects to adjust the volume of the chamber 10. In further embodiments, the shell 12 may include one or more elastomeric or movable walls that function as the movable member to adjust the volume of the chamber 10. In still further embodiments, the receiver 6 may include a combination of pistons, diaphragms, and/or movable walls that function as the movable member.

The illustrated movable member 8 of the illustrated embodiment is able to translate along the direction of the shared axis of the movable member 8 and the shell 12. This movement may be resisted by the friction forces resulting from the sliding of the o-rings 18 along the inner wall of the shell 12. This resistive force can be adjusted to the desired level by proper sizing of the o-rings 18 and the clearance gap between the shell 12 and the movable member 8.

With continued reference to the illustrated embodiment of FIGS. 2 and 3, the movable member 8 includes an offset surface 33 arranged to function as a stop for the movable member 8 against the first end cap 13. This ensures that the fluid flow path 9 is maintained between the ports 20, 21 even when the movable member 8 is at its limit of travel. Although the offset surface 33 is on the movable member 8 in the exemplary embodiment, it could additionally or alternatively be on the first end cap 13 and accomplish the same function.

The second end cap 14 includes one or more ports 23 (two are shown in FIG. 3) whereby fluid can be added to or removed from the chamber 11. In doing so, the fluid pressure within the chamber 11 may be regulated and, by extension, the fluid pressure within the chamber 10 may be also regulated. As one example, the port(s) 23 may be exposed to ambient pressure so that the pressure of air within the chamber 11 is approximately equal to the ambient pressure. As another example, one or more of the ports 23 may be fluidly connected to a regulated pressure source so that fluid pressure within the chamber 11 can be maintained at a relatively constant, elevated pressure.

When the pressure imbalance between the chambers 10, 11 results in a net force on the movable member 8 that exceeds the frictional resistive force of the o-rings 18 on the wall of the shell 12, then the movable member 8 will displace in the direction of the chamber 10, 11 having the lower pressure, unless and until the movable member 8 contacts one of the end caps 13, 14. Since the fluid chamber 10 is a portion of the total working fluid volume of the Rankine cycle system 1, changing the volume of the chamber 10 can result in a change in the working fluid pressure at the suction side of the pump 2.

When the system 1 is in a non-operating state, the temperature and pressure of the working fluid will be relatively constant throughout the entire working fluid volume of the system 1, and the charge of working fluid contained within that volume can exist as either a liquid, a vapor, or a two-phase liquid/vapor mixture, depending on the temperature and pressure. As the system 1 begins to operate, the addition of heat to the working fluid results in an increase of the system pressure at both the high pressure side (between the pump outlet and the expander inlet) and the low pressure side (between the expander outlet and the pump inlet).

The increase in pressure of the working fluid in the chamber 10 acting on the face of the movable member 8 causes the movable member 8 to move away from the first end cap 13 and towards the second end cap 14. This movement of the movable member 8 results in a volumetric increase of the chamber 10, and a corresponding volumetric decrease of the chamber 11. The increase in volume of the chamber 10 counteracts the increase in working fluid pressure, and the movable member 8 can be positioned at a new location within the receiver 6 such that the condensing pressure is again approximately equal to the reference pressure within the chamber 11. This may advantageously be used to operate the system 1 with a low condensing pressure in order to maximize the efficiency of the system 1 in converting heat energy to useful work.

When operation of the Rankine cycle system 1 is suspended, the working fluid cools down and, eventually, reaches the ambient temperature. In comparison to when the system is in operation, the working fluid will have a considerably greater liquid mass fraction during the non-operative condition. This shift from relatively low-density vapor to relatively high-density liquid causes a reduction in the pressure of the working fluid, which is compensated for by the movable member 8 repositioning itself to be nearer to the first end cap 13 in response.

If the charge of working fluid in the system 1 is sufficient to enable a working fluid pressure in the chamber 10 that is in equilibrium with the pressure in the chamber 11 without the movable member 8 reaching the limits of its travel (i.e. by contacting the first end cap 13 during a non-operative state, or by contacting the second end cap 14 during an operative state), then the receiver 6 will enable the Rankine cycle system 1 to maintain a desired working fluid pressure. In an especially preferable embodiment, the receiver 6 will be able to achieve the foregoing over the full range of ambient conditions and operating conditions.

Use of the receiver 6 may especially provide benefit when the working fluid used in the Rankine cycle system 1 is of a fluid type that has a sub-atmospheric saturation pressure at expected ambient temperatures. In a non-operating steady-state condition, a fraction of the working fluid charge will exist in a vapor state such that the system pressure is equal to the fluid saturation pressure corresponding to the fluid temperature. When the fluid drops to a low temperature, as may be experienced if the system is outdoors in cold weather, a greater portion of the working fluid will condense to liquid. If the total volume of working fluid is fixed, as would be the case in a system 1 lacking the variable volume receiver 6, this would leave only a small volume of vapor and, correspondingly, a low system pressure.

Working fluid pressures that are lower than the surrounding ambient pressure can be detrimental to the performance of the system. With positive pressure external to the system, infiltration of air or other non-condensable gases can occur at the piping joints along the system. The presence of these non-condensable gases will degrade the efficiency of the system during operation, as the working fluid partial pressure will be less than the full condensing pressure. This will result in a higher condensing pressure being required to achieve the same condensing temperature, thus reducing the pressure drop across the expander 3 and, consequently, reducing the system efficiency.

Incorporating the variable volume receiver 6 into the system 1 can help to alleviate this problem. The movable member 8 allows for the volume of the chamber 10 (and, consequently, the total working fluid volume) to vary. If the system 1 according to the invention were to be in a non-operating state in a low ambient temperature environment, the movable member 8 would displace to reduce the volume of the chamber 10 in response to the reduction in working fluid pressure. If the pressure in the chamber 11 were to be maintained at atmospheric pressure (such as by opening the port(s) 23 to ambient) then the working fluid pressure could be regulated to atmospheric pressure as well, thereby avoiding a pressure gradient that would drive non-condensable gases into the working fluid.

The desirability of such a Rankine cycle system 1 with regard to this aspect of the invention can be explained with reference to FIG. 6, which compares and contrasts the saturation pressure vs. temperature relationship of R245fa, a typical working fluid for organic Rankine cycles, to that of R134a, a typical refrigerant for vapor-compression air conditioning cycles. Whereas R134a does not reach a saturation temperature that is lower than the typical atmospheric pressure of 100 kPa until a temperature of −25° C. (an ambient temperature that is rarely if ever encountered in most habitable areas of the world), R245fa achieves the same at a much more moderate temperature of 16° C. When the variable volume receiver 6 is included in the system 1, the working fluid can be converted to be entirely liquid at a pressure equal to that of chamber 11 when the temperature is less than the saturation temperature at that pressure.

In order for the foregoing to be readily achievable, it is highly desirable that the system 1 has a proper charge of working fluid. In some embodiments, such a proper charge is an amount of working fluid that, when it is entirely in a liquid state, has a volume in excess of that which the system 1 would be capable of accommodating without the receiver 6. In this manner, the working fluid can be fully condensed to a liquid state while still having working fluid in the chamber 10. Since liquid fluids are essentially incompressible, the working fluid in such cases can be fully condensed (given a sufficiently low temperature) and can be able to maintain a pressure roughly equivalent to the pressure within the chamber 11.

As another aspect of the invention, the variable volume receiver 6 may provide advantages in charging the Rankine cycle system 1 with working fluid. In order to ensure a properly charged system, a regulated pressure can be applied to the chamber 11 as working fluid is introduced into the system 1. The regulated pressure applied to the chamber 11 may, for example, be a pressure greater than the saturation pressure corresponding to the temperature of the working fluid. Once the space available to the working fluid outside of the receiver 6 becomes filled, the working fluid begins to fill the chamber 10 and displaces the movable member 8. When the movable member 8 reaches a predetermined target position, the system 1 is properly charged. In some embodiments, the predetermined target position is a position that provides a suitable volume in the chamber 10 at a fully condensed liquid state.

The shell 12 may advantageously be constructed of a material that is at least partially transparent, so that the displacement of the movable member 8 can be directly observed. In this manner, a direct indication of the working fluid charge level and/or void fraction can be made available. As an example, the shell 12 can be constructed of a borosilicate glass. However, in other embodiments of the invention, displacement of the moveable member 8 can be indicated in other manners. For example, the position of the movable member 8 can be indicated using electronic sensors.

Turning to FIG. 4, an alternate embodiment of the receiver 6′ is shown. The shell 12 of the illustrated receiver 6′ is a metal cylinder that is directly and metallurgically attached to the end caps 13, 14 by welding, brazing, soldering, or the like. The tie rods 17 and o-rings 19 are not necessary in such a construction, and have been removed. An indicator ring 24 is provided to allow for an indication of the position of the movable member 8. The indicator ring 24 includes magnets 25 arranged along the circumference, with matching magnets arranged along the circumference of the movable member 8. The indicator ring 24 and the shell 12 may be constructed of non-ferrous metals so that the indicator ring 24 will travel along the receiver 6′ in response to the movement of the movable member 8, thereby indicating the position of the movable member 8 along the axis of the receiver 6′. In some embodiments, the indicator ring is constructed of an aluminum alloy, and the shell 12 is constructed of non-ferrous stainless steel, but other material combinations may be equally desirable.

In some embodiments of the Rankine cycle system 1, the receiver 6 is connected to a parallel-flow heat exchanger 24 including an integrated air-cooled condenser 5 and sub-cooler 7, as shown in FIG. 5. The illustrated heat exchanger 24 comprises a plurality of parallel arranged tubes 31 extending between first and second headers 29, 30. Corrugated fins 32 are arranged between adjacent ones of the tubes 31 in order to maintain proper tube to tube spacing and to provide enhanced heat transfer between the tubes 31 and air passing over the external surfaces of the tubes 31. Baffles (not shown) within the headers 29, 30 serve to separate the header internal volume into four separate manifolds, each of the headers 29, 30 having a first and a second manifold so that the condenser 5 includes those tubes 31 connecting the first manifold of the header 29 to the first manifold of the header 30, and the sub-cooler 7 includes those tubes 31 connecting the second manifold of the header 29 to the second manifold of the header 30.

A flow of working fluid enters the first manifold of the header 29 through fitting 25 as a superheated vapor, and passes through the condenser 5 to the first manifold of the header 30. The now liquid working fluid exits the header 30 through fitting 26, and is routed to port 20 of the receiver 6. Liquid working fluid is extracted from the receiver 6 through port 21, and is routed to fitting 27 connected to the second manifold of the header 30. The working fluid passes through the sub-cooler 7 to the second manifold of header 29, and is removed from the heat exchanger 24 through a fitting 28 connected to the second manifold of the header 29.

Various alternatives to certain features and elements of the present invention are described herein with reference to specific embodiments of the present invention. With the exception of features, elements, and manners of operation that are mutually exclusive of or are inconsistent with each embodiment described above, it should be noted that the alternative features, elements, and manners of operation described with reference to one particular embodiment are applicable to the other embodiments.

The embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art that various changes in the elements and their configuration and arrangement are possible without departing from the spirit and scope of the present invention. 

What is claimed is:
 1. A Rankine cycle system comprising: an expander; a pump; a condenser located along a working fluid flow path between an outlet of the expander and an inlet of the pump; and a receiver comprising a variable fluid volume at least partially defined by a movable member, wherein the variable fluid volume defines at least a portion of the working fluid flow path between the condenser and the inlet of the pump.
 2. The system of claim 1, further comprising a liquid sub-cooler located along the working fluid flow path between the receiver and the inlet of the pump.
 3. The system of claim 2, wherein the condenser and the liquid sub-cooler are parts of a single heat exchanger.
 4. The system of claim 1, wherein at least a portion of the variable fluid volume defines a cylindrical volume, the movable member defining an end of the cylindrical volume.
 5. The system of claim 4, wherein the movable member is movably disposed within the receiver so as to vary the length of the cylindrical volume.
 6. The system of claim 1, wherein the variable fluid volume is a first variable fluid volume, the receiver further comprising a second variable fluid volume at least partially defined by the movable member.
 7. The system of claim 6, wherein the second variable fluid volume is in fluid communication with the environment outside of the receiver to maintain a substantially atmospheric pressure within the second variable fluid volume.
 8. The system of claim 6, wherein the second variable fluid volume is in fluid communication with a regulated pressure source.
 9. The system of claim 1, wherein the movable member is positioned within the receiver, and wherein the receiver includes a position indicator to indicate the location of the movable member within the receiver.
 10. The system of claim 9, wherein the position indicator is magnetically coupled to the movable member.
 11. The system of claim 1, wherein the variable fluid volume is further defined by an end face of the receiver, the end face including first and second ports in fluid communication with the variable fluid volume and defining portions of the working fluid flow path immediately upstream and downstream of the variable fluid volume.
 12. The system of claim 11, wherein at least one of the end face and the movable member includes an offset surface so that a fluid flowpath is maintained between the first and second ports when the offset surface is in contact with the other of the end face and the movable member.
 13. A method of charging a Rankine cycle system with working fluid, comprising: applying a regulated pressure to a chamber located within a receiver in the Rankine cycle system; introducing the working fluid to the Rankine cycle system, the working fluid being separated from the chamber by a movable member of the receiver; monitoring displacement of the movable member; and stopping the introduction of working fluid into the Rankine cycle system when the movable member reaches a predetermined position.
 14. The method of claim 13, wherein the regulated pressure is greater than a saturation pressure corresponding to a temperature of the working fluid.
 15. The method of claim 13, wherein monitoring the displacement of the movable member includes at least one of visual observation of the movable member and electronic measurement of the movable member.
 16. The method of claim 13, wherein monitoring the displacement of the movable member includes monitoring the displacement of a position indicator magnetically coupled to the movable member.
 17. A method of pressure adjustment within a Rankine cycle system having an expander and a pump, the method comprising: changing a pressure of working fluid within the system between the expander and the pump; changing a volume of an internal chamber of a receiver in fluid communication between the expander and the pump responsive to changing the pressure of the working fluid within the system.
 18. The method of claim 17, wherein changing the volume of the internal chamber comprises moving a portion of the receiver.
 19. The method of claim 17, further comprising receiving the working fluid into the receiver from a first heat exchanger portion, and discharging the working fluid from the receiver to a second heat exchanger portion.
 20. The method of claim 17, further comprising changing a volume of a second internal chamber in the receiver responsive to changing the pressure of the working fluid within the system. 